1. Field of the Invention
The present invention relates to a catalytic conversion process, and particularly to a process for olefin epoxidation with the co-production of a nylon precursor.
2. Background of the Art
Epoxidation of olefin is a well-established reaction, which usually requires heterogeneous catalysts containing transition metals. An early version of commercial catalyst is titanium supported on amorphous silica (cf. R. A. Sheldon, I. WL. C. E. Arends, H. E. B. Lempers, Catal Today 1998, 41, 387–407). If used in an aqueous system the catalytically active component, titanium, leaches out; and the catalyst loses catalytic activity proportionate to the loss of titanium content. Thus, organic oxidants such as tert-butyl hydroperoxide have been used. However, use of tert-butyl hydroperoxide in an organic system leads to the production of alcohol, i.e., tert-butanol, as a reaction byproduct. Reprocessing of this alcohol to its hydroperoxide significantly adds to the production cost of the epoxide.
In 1983 Taramasso et al showed a crystalline microporous titanosilicate (TS-1), isostructural to ZSM-5, exhibited high catalytic activity for olefin epoxidation in either organic or inorganic reaction systems (M. Taramasso, G. Perego, B. Notari, U.S. Pat. No. 4,410,501). While virtually all titanium catalysts leach when exposed to an aqueous environment, the titanium active species in TS-1 is one of the most stable in this regard. TS-1 opened a new area to use hydrogen peroxide as an oxidant, which gives water as a byproduct. Here, this epoxidation process is more environmentally friendly. However, crystalline microporous TS-1 and other transitional metal-containing zeolites have small pores (normally less than 1.2 nm in diameter), which prevent access of some important, bulky reactants to the active sites. For certain large reactants, a catalyst with large pores is required to have good catalytic performance.
Transition metal-containing mesoporous materials (i.e. having pore diameters between 1.5 and 30 nm) have been disclosed, such as Ti-MCM-41 and Ti-MCM-48 (Pinnavaia et al. J. Am. Chem. Soc., 1996, 118, pgs. 9164–1971). These materials have unique pore structures: Ti-MCM-41 possesses one-dimensional pores that are regularly arranged in parallel, whereas Ti-MCM-48 has three-dimensional, ordered pores.
Now, a new mesoporous material (denoted as TUD-1, U.S. Pat. No. 6,358,486 B1) has been disclosed, having a three-dimensionally interconnected pores system. This pore system has advantages over that of MCM-41 because it facilitates mass transfer of reactants and products and reduces the possibility of pore blockage. This mesoporous material can be functionalized by adding transition metals and can then be used as a catalyst for epoxidation.
The TUD-1 mesoporous material mentioned above has an amorphous character. Other, amorphous Ti materials generally have the above-cited leaching problem. Use of an organic medium rather than water can minimize metal leaching, but causes another problem, i.e., formation of undesirable byproduct alcohols. What is needed is a process to effectively utilize the corresponding byproduct alcohol as a valuable product and to permit the use of bulky reactants and organic oxidants for epoxidation without net generation of a co-product alcohol.